2, Wageningen University, Wageningen, , Netherlands
One way to overcome the theoretical limit for the efficiency of single junction photovoltaics (PV) is to reduce the quantum defect via multiple exciton generation. Singlet exciton fission in tetracene and other polyacenes has been proven to be a highly efficient pathway to generate two triplet excitons from one singlet exciton. If those triplet excitons could be transferred to silicon, the efficiency of conventional Si solar cells could be dramatically improved. Triplet excitons in tetracene have an energy of 1.25 eV which is slightly higher than the bandgap of Si (1.1 eV), a requirement for triplet exciton energy transfer.
To enable transfer of triplet excitons, we functionalize a Si (111) surface with different benzene derivatives to control the alignment of the tetracene on the Si surface, keep the Si surface oxide free and allow for a close distance of ~1 nm between tetracene and Si. The energy of triplet excitons can transfer only via Dexter energy transfer, which requires the wavefunction of initial and final states to overlap, so aligning the wavefunction of Si and tetracene will allow to improve the energy transfer rate dramatically.
We study the energy transfer from the dependence of the decay rate on tetracene thickness. Tetracene growth in islands on the silicon surface, where each island has a different thickness. We correlate the thickness of the tetracene islands with the lifetime of the triplet excitons for each island individually. We find that the delayed photoluminescence (PL) decay lifetimes of the thicker islands is longer. This means that the triplet excitons that experience the interface during their lifetime are quenched, consistent with energy transfer into silicon.